**5.4 Volatile fatty acids**

The accumulation of the volatile fatty acids (VFA) during the non balance of the process reflects directly an uncoupling kinetic between the acid producers and consumers (Hickey & al., 1989). The concentration of VFA was suggested for the control and the monitoring of the anaerobic digester (Hill & Bolte, 1989). The VFA is generally measured by gas chromatography (GC) with the use of a detector with ionization of flame (FID), to obtain the individual VFA, or by titration which gives the concentration of total VFA, and which is less expensive and is largely used at the commercial biogas plants. Several methods of titration for the determination of total VFA were proposed, for example a simple titration (Delbès, 2000), a titration at 5-point, and a titration at 8-point.

However, several studies specified that the individual VFA can provide more significant information concerning an early failure of the process the failure of process (Nielsen, 2006).

Production of Biogas from Sludge Waste and Organic Fraction of Municipal Solid Waste 161

• The growth kinetics of bacteria is low, the pretraitement kinetics is also low and the

• the microbial populations are sensitive to the disturbances, in particular with oxygen

• the treatment by anaerobic digestion is often insufficient to directly reject the effluents in the natural environment: an aerobic postprocessing is necessary to complete the

As mentioned previously, the temperature and substrate compostion have a high effect on biogas production. Their effects are confirmed by the study published by several researchers.

The obtained values of different parameters of the (co)-digestion experiments under mesophilic and thermophilic conditions are presented (see figure 2, 3, 4, 5, 6). It should be noted that the volume of the mesophilic co-digester which is 2000 m3 is 20 times larger than pilot scale digesters 500 l. Therefore, the absolute biogas volume produced is different from the other cases and no comparison can be made. However, a comparison for different

Figure 2 represents daily average biogas production values for the four studied cases where the thermophilic co-digestion shows the best results. Eventhough temperature has a certain effect on biogas production, adding solid waste is a contributing factor to this production. In

The use of gas production rate GPR as a comparison parameter led us to include the data from the industrial scale digester. The results shown on Figure 3 confirm that the combined effect of temperature and solid waste addition is positive and considerable. Moreover, thermophilic co-digestion presents the best GPR results wich are confirmed by values of

**8. Effect of temperature and substrate composition on biogas production** 

As an example the results, obtained by Derbal et al. (Derbal & al., 2011) can be cited.

A.D. T=35°C A.D. T=35°C A. Co-D. T=55°C

However, it has also certain disadvantages:

and with heavy metals (OTV, 1997);

parameters is presented as follows:

**GP (m3/j)**

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

fact, solid wastes contain a high percentage of organic matter.

Fig. 2. Comparison of gas production (GP)

• Significant capital costs ;

• High sensitivity to the toxic compounds (Schnurer & al., 1999); • A slower degradation compared with aerobic processes (Bitton, 1994);

elimination of carbon and possibly nitrogen and phosphorus.

time needed for the treatment is relatively long;
